Screen printing is accomplished by a squeegee being moved under pressure (force) across the screen which deflects the screen downward into momentary contact with a substrate and forces the ink through orifices in the screen mesh. The interaction of the screen mesh, the substrate, ink, and squeegee results in the ink contacting the substrate and the ink shearing from the screen mesh onto the substrate.
Screen printing is distinct from most other forms of printing in that it is an "off contact" form of printing. Lithographic, flexographic, rotograuve, and roto screen printing are forms of "on contact" printing in which the ink is transferred to the substrate by contact with a rotating drum or cylinder. Many of the problems overcome by the invention described below do not exist in "on contact" printing due to the generally rigid rotating cylinder or drum in "on contact" printing.
In addition to the distinction discussed above, another distinction with roto screen printing is that in screen printing it takes two steps to get the ink through the screen to the substrate. The first step is the placement of the ink across the screen and into the orifices. This step occurs prior to the deflection of the screen by the squeegee. This placement of the ink is accomplished using a flood bar such as described in U.S. patent application Ser. No. 08/371,732, which is incorporated by reference. While the placing of the ink in the orifices of the screen mesh by the flood bar affects both the quality and the speed of printing, a detailed understanding of the workings of the flood bar is not required to understand the current invention. The flood bar will be discussed only briefly in the detailed description of the invention. With a brief understanding of the distinctions of screen printing from other forms of printing and an understanding of the flooding process, the background with respect to the specific invention is addressed.
The relationship between the screen mesh, the substrate, the ink, and the squeegee is detailed more completely by explaining the screen printing process. The screen mesh is positioned some distance above the substrate. Therefore, the screen is not in contact with the substrate (i.e. off-contact). The distance between the screen and the substrate is defined as the off-contact distance. The position of the screen mesh relative to the substrate is not a random occurrence. Several factors influence the exact placement of the screen mesh above the substrate, (i.e., the off-contact distance) in order to achieve higher quality and faster speed operation. At this time, it is recognized that the most relevant factor is the screen mesh's tension.
In recent years, the tension placed on the screen mesh has increased from a range of seven newtons per centimeter to that of eighty-five newtons per centimeter and higher. The development of higher screen tensions has resulted in the screen mesh being able to be placed in closer proximity to the substrate. Because of this higher tension and the closer placement of the screen mesh to the substrate, image distortion is greatly eliminated, interface friction and pressure between the screen mesh and the squeegee or flood bar in the flooding process is reduced, more uniform interface pressure is achieved between the squeegee and the screen mesh, as well as other benefits. One of the reasons that tension in the screen mesh results in these benefits is that a more uniform pressure is required to deflect the screen and thus the amount of deflection (i.e., the off contact distance) required is reduced as explained in more detail in the detailed description. Although the increased tension minimizes problems of non-uniform pressure, the interface pressures will never be uniform and there will always be an off contact distance, as explained in greater detail in the detailed specification.
The primary purpose of the squeegee is to deflect the screen mesh into contact with the substrate and to apply a downward hydraulic force onto the ink and let the tension of the screen mesh snap the screen mesh away from the substrate shearing the ink from the mesh therein depositing the ink onto the substrate.
The squeegee is shaped such that the engagement of a tip of the squeegee against the screen mesh is a line contact. The amount of force exerted by the tip of the squeegee against the screen mesh effects the deflection of the screen mesh. The force needed on the squeegee to deflect the screen is dependent on the screen tension (See FIG. 4). The amount of force needed to deflect the screen mesh is not uniform from the middle of the screen mesh engaged by the squeegee to the edge of the screen mesh engaged by the squeegee, or from the middle of the stroke to both the beginning and end of the stroke. The increased tension just minimizes the problem.
The screen/squeegee interface affects the pressure the ink receives from the squeegee to force the ink through the screen mesh into engagement with the printing substrate. Interface pressures along with the squeegee speed control the shear rate of the ink from the orifices of the mesh. While the screen mesh is in a position of being stretched downward to the substrate, the ink interface pressure exists in the dynamic state. As the squeegee moves forward, the screen mesh snaps up vertically, participating in the ink shear process. The hydraulic pushing on the ink additionally is controlled by the force on the mesh pushing upward contacted by the constance of a squeegee pressing downwards. Therefore, the more consistent the squeegee is, the more reproducible the results.
Conventional squeegees are made of a polymeric material. The amount of force at the screen/squeegee interface is the result of the angle of the squeegee, the height or cantilever length of the squeegee, the material proprieties of the squeegee such as durometer of the material, and amount of force exerted on the squeegee. The moving under pressure of the squeegee on the screen and the chemical reaction between the ink and the squeegee results in the squeegee both physically wearing away and the molecular properties of the squeegee changing, such as durometer, chemical (ink) resistance, and elastic limit point. These changes result in the screen/squeegee interface pressure varying during screen printing operation. The varying pressure results in varying the ink delivery characteristics causing smudges and inconsistences in ink quality and quantities. Depending on the ink being used, the squeegee could be required to be changed as much as up to every hour to ensure proper quality in very high quality work.
While the squeegee can be ground to achieve the proper tip, this grinding must be precise. Furthermore, the grinding results in the height, also referred to as cantilever length, of the squeegee being reduced. The change in the height of the squeegee results in the characteristics of the screen/squeegee interface changing. Since the stiffness is proportional to the cube of the height, a small change in length affects the stiffness greatly. Furthermore, the grinding of the tip does not remove the entire portion of the squeegee which has had molecular properties degraded as the result of interaction with the ink.
It is desired to have a squeegee wherein the forces at the screen/squeegee interface are held constant.